1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use in a communication apparatus that is used in high frequency bands, and more particularly to a ladder type surface acoustic wave filter.
2. Description of the Related Art
Generally, as a surface acoustic wave filter designed to achieve good characteristics such as low loss and wide band characteristics, a ladder type surface acoustic wave filter that has one-port resonators alternately arranged on a series arm and parallel arms is known. FIG. 1 is a schematic view of a ladder type surface acoustic wave filter as described above. In a surface acoustic wave filter 110 shown in FIG. 1, two surface acoustic wave resonators 111a and 111b are connected to define a series arm, and three surface acoustic wave resonators 112a, 112b, and 112c are connected to the series arm to define parallel arms, respectively. Each of these surface acoustic wave resonators 111a, 111b, 112a, 112b, and 112c includes an interdigital transducer 124 having a plurality of electrode fingers 125, and a pair of reflectors 122 having a plurality of electrode fingers 123 which are provided on both sides of the interdigital transducer 124.
In the field of communication apparatuses using the above-described surface acoustic wave filter, high-frequency requirements have been increasing, and development of new surface acoustic wave filters meeting the increased requirements are progressing. For example, Japanese Unexamined Patent Publication No. 9-167936 discloses a 38-to-46-degree Y-cut X-propagation LiTaO3 substrate to meet the high-frequency requirements. Conventionally, as a substrate of the surface acoustic wave filter, a 36-degree Y-cut X-propagation LiTaO3 substrate has conventionally been used because it produces a low propagation loss and has a large electromechanical coupling coefficient.
Although the propagation loss decreases where the thickness of an electrode film defining the interdigital transducers is negligibly small relative to the wavelength of a surface acoustic wave, the 36-degree Y-cut X-propagation LiTaO3 has a problem in that, the propagation loss increases where the thickness of an electrode film is increased. Particularly, as the wavelength of the surface acoustic wave decreases in the high-frequency band, the thickness of the electrode film relative to the wavelength becomes so large that the propagation loss increases. On the other hand, when the influence of bulk waves and the increase in electrode resistance are taken into account, reduction in thickness of the electrode film is not preferable because it causes reduction in the characteristics.
In view of the foregoing problems, Japanese Unexamined Patent Publication No. 9-167936 discloses that even in a case where the thickness of the electrode film is increased in consideration of the influence of bulk waves and the increase in the electrode resistance, the propagation loss can be reduced by use of the 38-to-46-degree Y-cut X-propagation LiTaO3 substrate as a substrate of the surface acoustic wave filter.
Conventionally, as a modulation method for cellular phones, a TDMA (time division multiple method) has been used. Recently, however, a CDMA (code division multiple method) is used to efficiently transmit an increasing amount of information. In ordinary cellular phone systems, the total-system frequency band is divided via channels into smaller bands. In this case, according to the TDMA method, the frequency width per channel is as small as several tens of kilohertz (kHz). However, according to the CDMA method, the frequency width is as large as 1 MHz or more.
Where very small ripples exist in the passbands, the difference in the frequency width per channel according to the aforementioned modulation methods becomes apparent with the difference in influence of the ripples. Specifically, according to the TDMA method, when very small ripples exist in the passbands, deviation in loss does not increase since the per-channel frequency width is relatively small. However, according to the CDMA method, the deviation in loss increases since the per-channel frequency width is relatively large. In the cellular phone system, a large amount of loss makes modulation difficult. With a large amount of the deviation in loss that diffuses the frequency for information, a problem also arises in that the CDMA method itself makes modulation difficult. Therefore, with the CDMA method, very small ripples occurring in the passbands become apparent as a problem while such ripples have not caused a problem in the TDMA method. In particular, the very small ripples are required to be reduced to be less than 0.7 dB.
Nonetheless, in the conventional ladder type surface acoustic wave filter, ripples of more than 0.7 dB have occurred in the passbands because of interference of reflection caused in the interdigital transducers and interference of reflection caused in the reflectors in the series arm surface acoustic wave resonators.
Hereinbelow, a description will be given regarding reasons why the ripples are caused in the passbands. The description will be provided referring to the reflector as an example, but the description can also be applied to the interdigital transducers.
Each of FIGS. 2 and 3 shows frequency characteristics of the reflector. FIG. 2 shows the characteristics where the number of the electrode fingers is 50, while FIG. 3 shows the characteristics where the number of the electrode fingers is 100. In either of the figures, the center frequency is 800 MHz.
As shown in FIGS. 2 and 3, outside of the stopband, the minimum value to which the reflection coefficient becomes small is repeated. With these minimum values, since excitation efficiency decreases, in view of impedance characteristics of the surface acoustic wave resonator, locally-high-impedance portions occur, as shown in FIG. 4. A surface acoustic wave resonator having the characteristics in which the aforementioned locally-high-impedance portions occur is series-connected as shown in FIG. 5, and transmission characteristics relative to the frequency are measured. As a result, it is known that very small ripples as shown in FIG. 6 occur. In FIG. 6, a graph indicated by B is an enlarged view of a graph indicated by A, and scale points thereof are indicated on the right side of the vertical axis (other characteristic views in this Specification are similarly presented). As shown in FIG. 6, when the ripples occur in the transmission characteristics of the series-connection configuration, ripples also occur in filter characteristics of a surface acoustic wave filter configured using the aforementioned surface acoustic wave resonator. That is, by the influence of the minimum values, ripples occur in filter characteristics of the surface acoustic wave filter.
Hereinbelow, a description will be given of frequencies having the aforementioned minimum values of the reflection coefficients.
Expression 1 shown below can be used to regulate frequencies f having the minimum values of the reflection coefficients by a center frequency f0.
f/f0=(1xe2x88x92K11/k0)xc2x1{(K12/k0)2+(n0/N)2}xc2xd
In the above, K11 and K12 represent, respectively, a self-coupling coefficient (coefficient representing the coupling strength between surface acoustic waves proceeding in the same direction), which is uniquely determined according to factors such as substrate material and electrode-film thickness, and a mutual coupling coefficient (coefficient representing the coupling strength between surface acoustic waves proceeding in directions opposing each other); k0 represents the number of waves in the center frequency; n0 represents an integer larger than 0; and N represents the number of the electrode fingers.
As shown in the expression that expresses the frequency having the minimum value, if the number N of the electrode fingers is infinite, (n0/N)2=0; however, if the number of the electrode fingers is finite, (n0/N)2 cannot be neglected, and the minimum value exists for each value n0 (integer larger than 0). That is, an innumerable number of the minimum values of reflection coefficients exists outside of the stopband. In addition, since K11, K12, and K0 are determined to be constants in design, differences in frequency for the minimum values of the reflection coefficients are determined according to the number of the electrode fingers. In this case, the fewer the number N of the electrode fingers, the larger the difference in the frequency. Clearly, from comparison between FIGS. 2 and 3, the differences in the frequency in FIG. 2 showing the case where the number of the electrode fingers is relatively small increase. Also, from comparing FIGS. 2 and 3, the minimum values of the reflection coefficients in FIG. 2 showing the case where the number of the electrode fingers is relatively small are even smaller than those in FIG. 3 which shows the case where the number of the electrode finger is relatively large.
In the surface acoustic wave filter conventionally used, since the number of the electrode fingers is not taken into consideration, the minimum value of the electrode fingers in one of the series arm surface acoustic wave resonators is the same as the minimum value of the reflection coefficient in the other one of the series arm surface acoustic wave resonators. Therefore, the effects of the two resonators are increased by each other, thereby producing larger ripples occurring in the surface acoustic wave filter.
One reason that can be considered for occurrence of the enlarged ripples is that the propagation loss is reduced by use of 38-to-46-degree Y-cut X-propagation LiTaO3 substrate. Specifically, a no-load Q value (a parameter representing sharpness in resonance) increases, thereby increasing sharpness of the ripples occurring because of, for example, the aforementioned interdigital transducers. As shown in FIG. 7, ripples increase when the cut angle is greater than 36 degrees.
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter, a duplexer, and a communication apparatus, wherein the size of ripples is reduced without reducing the effects of a substrate used for reducing a propagation loss, and therefore, the ripples in the pass bands are smoothed.
According to a preferred embodiment of the present invention, a surface acoustic wave filter includes a Y-cut X-propagation LiTaO3 substrate having a cut angle of about 38 degrees to about 46 degrees, at least two surface acoustic wave resonators on a series arm and at least one surface acoustic wave resonator on a parallel arm, and either the number of pairs of electrode fingers or the number of electrode fingers that configure the surface acoustic wave resonators on the series arm is appropriately set. Specifically, with reference to one of the surface acoustic wave resonators that has either the smallest number of pairs of the electrode fingers or the smallest number of the electrode fingers, either the number of pairs of the electrode fingers or the number of the electrode fingers in at least one of the other surface acoustic wave resonators on the series arm is set so as not to be a positive integer multiple of the number in the former case. As a result, ripples caused by the series arm surface acoustic wave resonator having either the smallest number of pairs of the electrode fingers or the smallest number of the electrode fingers (since either the number of pairs of the electrode fingers or the number of the electrode fingers is smallest, the size of ripples caused thereby is largest) are suppressed and minimized by the series arm surface acoustic wave resonator in which either the number of pairs of the electrode fingers or the number of the electrode fingers is not a positive integer multiple thereof.
Other features, characteristics, elements and advantages of the present invention will become apparent from the following description of preferred embodiments thereof with reference to the attached drawings.